Added uninstall script

Mainly just a reversal of install.sh, but more transactional - will abort if either SHARE_PATH or BIN_PATH does not exist.

Not tested yet, though.  I thought I had installed ruby-build as standalone, but when I went to uninstall it with the script, SHARE_PATH wasn't there, and none of the ruby-build/bin/* files were in BIN_PATH.  

Will test a little later in a VM, but wanted to submit for feedback now anyway.